The present invention relates to a method and apparatus for multiple sheet detection suitable for a sheet-fed press.
A multiple sheet detector is used to perform multiple sheet detection when sheets (paper) are supplied to a conventional sheet-fed press.
More specifically, when a sheet is to be supplied from a sheet feeder to a sheet-fed press (to be referred to as a printing press hereinafter), a leading edge of a sheet is brought into contact with a stopper called a front gauge arranged at the press-side distal end portion of a stack board, and then the sheet is fed to the printing press. In this case, in order to prevent simultaneous feeding of two or more sheets, a light-emitting element is arranged on the lower surface of the stack board near the front gauge, a through hole is formed at a predetermined portion of the stack board opposite to a light-emitting portion of the light-emitting element, and a light-receiving element is arranged on the upper surface of the feed board at a position opposite to this through hole. More specifically, light emitted from the light-emitting element is projected in a direction of thickness of a sheet to be fed, light transmitted through the sheet is received by the light-receiving element, and an output from the light-receiving element is converted into an electrical signal. An output level corresponding to a light amount represented by the electrical signal is compared with a predetermined discrimination level, thereby performing multiple sheet detection on the basis of a comparison result.
In such a multiple sheet detector, use or non-use of a multiple sheet detection function for sheets can be switched upon an operation using a switch or the like. More specifically, it is often desirable to perform printing without using the multiple sheet detection in accordance with the types of sheets and printing conditions. In order to cope with this need, the use or non-use of the multiple sheet detection can be switched.
In the conventional multiple sheet detector, however, when the multiple sheet detection function is set in a non-use state in previous printing, and an operator forgets to switch the function to the use state, the next operator may often forget to set the use state of the multiple sheet detection function in current printing.
When the operator forgets to set use of the multiple sheet detection function and printing is started, although multiple sheet feeding can be detected by the standard multiple sheet detection function, multiple sheet detection cannot be performed and multiple sheet feeding cannot be prevented.
When the operator forgets to set use of the multiple sheet detection function and printing is started, although multiple sheet feeding can be detected by the standard multiple sheet detection function, no alarm is generated in feeding of two or more sheets. Therefore, a countermeasure against multiple sheet feeding is delayed since the operator is not informed of this multiple sheet feeding.
Feeding of two or more sheets in the printing process is one of the major drawbacks. If this drawback occurs, normal printed products cannot be obtained, or printing is interrupted, thereby delaying the printing process. When printing continues while the operator does not notice that the multiple sheet detection function is not set, blank sheets may be mixed in the printed products, or a failure may occur in the printing press itself. This may lead to a claim for defective delivered products at a later date.